FIG. 1 depicts a typical conventional magnetic element 10. The conventional magnetic element 10 can be used in non-volatile memories, such as a magnetic random access memory (MRAM). The magnetic element 10 typically includes a reference layer 14, a spacer layer 16, and a free layer 18. The free layer 18 has a magnetization having an equilibrium position along an easy axis 19. The easy axis can be inside the film plane or out of the film plane. Other layers, such as antiferromagnetic pinning layers 12 might also be provided. Moreover, other similar structures, such as dual magnetic tunneling junctions (MTJs) and MTJ-Spin Valve dual structures with spacer and/or barrier layers at both sides of the free layer, might be used. For clarity only single reference layer 14, spacer layer 16 and free layer 18 are plotted. The reference layer 14 and free layer 18 are typically ferromagnetic, but may be multilayers including both ferromagnetic and non-ferromagnetic layers. The spacer layer 16 is nonmagnetic and may be insulating (for example in a magnetic tunneling junction) or conductive (for example in a spin valve). The difference in orientations of the magnetizations of the free layer 18 and the reference layer 14 determines the resistance of the conventional magnetic element 10 and thus the state of the conventional magnetic element 10. When the free layer and the reference layer are substantially parallel the magnetic element 10 has a resistance R, when the free layer and the reference layer magnetization are substantially antiparallel the magnetic element 10 has a resistance R+ΔR. The resistance difference, ΔR, can be positive or negative depending on the materials chosen for the different layers of the magnetic element 10. The state of the magnetic element can be determined by measuring its resistance.
The conventional magnetic element 10 may be programmed either using a magnetic field or a current driven through the magnetic element (e.g. using the spin transfer effect). FIG. 2 depicts the conventional magnetic storage cell 20 that is used in a memory and programmed using a conventional magnetic field switching method. Referring to FIGS. 1 and 2, the magnetic element 10 is part of a conventional magnetic storage cell 20 that also includes a selection transistor 22. Also depicted are a corresponding bit line 24 connected to the conventional magnetic element 10, a read word line 26 connected to the selection transistor 22, a source line 28 connected to the selection transistor 22 and a write word line 30. In field switching, the bit line 24 and the word line 30 are activated, currents ib and iw are driven through the bit line 24 and the write word line 30, respectively. Typically, the bit line 24 and the write word line 30 are mutually perpendicular, as shown in FIG. 2. During such a write operation, the selection transistor 22 is disabled to preclude current from being driven through the conventional magnetic element 10. The currents ib and iw produce two magnetic fields, HB and HW, respectively. The conventional magnetic element 10 of the storage cell 20 in the region where the bit line 24 and write word line 30 cross (the “crosspoint”) experiences both magnetic fields. Consequently, the magnetization of the free layer 18 of the conventional magnetic element 10 may switch directions. The conventional magnetic element 10 is, therefore, programmed. Other conventional magnetic elements (not shown) for other magnetic storage cells (not shown) in a memory array only experience the field from at most one of the active bit line 24 and the active write word line 30. Consequently, other magnetic storage cells should not be switched.
Although the conventional magnetic element 10 can be programmed as depicted in FIG. 2, programming utilizing field switching has significant drawbacks. These drawbacks may be particularly severe for higher memory densities. For example, the field switching mechanism in a high density memory array suffers from a half selection problem. In particular, a conventional magnetic element 10 that only experiences the magnetic field HB or HW from either the bit line 24 or the write word line 30, but not both, may be accidentally switched. Undesired writing may thus occur. Furthermore field writing may not be easily scaled to higher densities due to the complicated cell structure. When the conventional magnetic element 10 is reduced in size, larger magnetic fields may be required to write to the conventional magnetic element 10. Consequently, larger currents ib and iw may be needed to program the conventional magnetic element 10. The operation current is typically greater than ten milliamps, which is significantly larger than desired.
In another method, the magnetization state of the magnetic elements 10 may also be switched using the spin transfer effect. Spin transfer based switching is desirable because spin transfer is a localized phenomenon that may be used to write to a cell without inadvertently writing to neighboring cells. Consequently, it would be desirable to use the conventional magnetic elements 10 in a magnetic memory, such as MRAM, that employs spin transfer switching.
FIG. 3 depicts a conventional magnetic storage cell 40 including the conventional magnetic element 10 and residing in a memory that uses spin transfer based programming. The conventional storage cell 40 includes a conventional magnetic element 10 that is typically an MTJ and a selection transistor 42. Also depicted are a conventional bit line 44, a conventional word line 46, and a conventional source line 48. Referring to FIGS. 1 and 3, this switching mechanism for the conventional magnetic storage cell 40 uses a current im passing perpendicular to the planes (in the z-direction in FIG. 1) of the layers of the conventional magnetic element 10. When the selection transistor 42 is activated by the conventional word line 46 and a current is passed through the magnetic element 10, the electrons carrying the current become spin-polarized by the local magnetization. The polarized electrons exert a torque on the magnetization of the free layer 18. This spin transfer torque may generate spin waves and/or complete switching of the magnetization of the free layer 18 when the current im reaches a critical value ic. When the current is driven from the free layer 18 to the reference layer 14, the spin transfer torque can switch the free layer magnetization to be parallel to the conventional reference layer 14 magnetization. When the current is driven in the opposite direction, the spin transfer torque is also in the opposite direction and can switch the magnetization of the free layer 18 to be anti-parallel to that of the conventional reference layer 14. Thus the magnetic element 10 can be programmed. During reading a read current is passed through the magnetic element 10 to sense the resistance of the magnetic element 10 that depends on the relative orientation of the magnetizations of the free layer 10 and of the reference layer 14. Note that the read current should be less than the current used in writing in order to avoid in advertently writing to the magnetic element 10.
Programming the magnetic storage cell 40 using the spin transfer effect has benefits. In particular, the spin transfer effect is a more localized phenomenon. Thus, the conventional magnetic storage cell 40 may not suffer from the half selection problem. In addition, such technology is characterized by good scalability. Stated differently, a magnetic memory using the magnetic storage cell 40 is easier to scale to higher densities. Moreover, the switching current required to program the magnetic element is relatively small. For example, in some applications, the write current needed for switching the free layer of the magnetic element 10, or the critical current ic, is less than approximately one milliamp.
Although use of the spin transfer effect has significant benefits, one of ordinary skill in the art will recognize that there are also drawbacks. For the sizes of the magnetic element 10 at which spin transfer switching becomes important, even the low critical current of approximately one milliamp corresponds to a current density of approximately 106˜107 A/cm2. Such a large current density passing through the conventional magnetic element 10, particularly when an insulating spacer layer 16 is used, may adversely affect the reliability of the magnetic storage cell 40. For example, the large current density may damage or break down the insulating spacer layer 16. Moreover, such a large current density requires a large selection transistor 42. The large selection transistor 42 required to provide this large current density may limit the density of a memory array formed using the magnetic storage cell 40.
To address problems in the spin transfer based switching scheme, magnetic field assisted current switching has been proposed. In one such scheme, referring to FIG. 3, the bit line can be arranged so that the current in the bit line 44, ib, which is also driven through the magnetic element 10, generates a magnetic field Hb that assists in switching the magnetic element 10. The use of the magnetic field Hb reduces the critical current. However, the bit line current is relatively small ib=im, on the order of one milliamp or less. Consequently, the strength of the field used in assisting switching is small, providing very little assistance in switching the magnetic element 10.
FIG. 4 depicts a magnetic storage cell 40′ in another such conventional magnetic field assisted current switching scheme. The components depicted in FIG. 4 that are analogous to those depicted in FIG. 3 are labeled similarly. Consequently, storage cell 40′, a selection transistor 42′, a bit line 44′, a word line 46′ and a source line 48′ are shown. In addition, a conventional field line 50 is depicted. A current, iA, is driven through the conventional field line 50 that generates a field, HA, that assists in switching the state of the conventional magnetic element 10. In similar schemes, the conventional field line 50 may be perpendicular to the conventional bit line 44′ and/or below the magnetic element 10. In such scheme a large magnetic field could be generated. However, the extra, conventional field line 50 is used. The fabrication of the conventional field line 50 increases the manufacturing complexity and may adversely affect the density of a memory using such lines 50.
Accordingly, what is needed is a system and method for providing a magnetic memory element that can be switched using a lower current density. The present invention addresses such a need.